CHAPTER XXII. 

 DEVELOPMENT OF THE ORGANS OF MOTION. 



The Motive Apparatus of Vertebrates. These are constituted by the 

 Passive and Active Organs of Motion (Skeleton and Mnscles). The 

 Significance of the Internal Skeleton of Vertebrates. Structure of the 

 Vertebral Column. Formation and Number of the Vertebrae. The Ribs 

 and Breast-bone. Germ-history of the Vertebral Column. The Noto- 

 chord. The Primitive Vertebral Plates. The Formation of the Meta- 

 mera. Cartilaginous and Bony Vertebrae. Intervertebral Discs. 

 Head-skeleton (Skull and Gill-arches) .Vertebral Theory of the Skull 

 (Goethe and Oken, Huxley and Gegenbaur). Primitive Skull, or 

 Primordial Cranium. Its Formation from Nine or Ten Coalescent 

 Metamera, The Gill-arches (Bibs of the Head). Bones of the Two 

 Pairs of Limbs. Development of the Five-toed Foot, adapted for 

 Walking, from the Many-toed Fin of the Fish. The Primitive Fin 

 of the Selachians (Archipterygium of Gegenbaur). Transition of the 

 Pinnate into the Semi-pinnate Fin. Atrophy of the Rays or Toes 

 of the Fins. Many-fingered and Five-fingered Vertebrates. Com- 

 pnn'son of the Anterior Limbs (Pectoral Fins) and the Posterior Limbs 

 (Ventral Fins). Shoulder Girdle and Pelvis Girdle. Germ-history of 

 the Limbs. Development of the Muscles. 



" In forming his estimate of my entire theory, the reader may begin with 

 the details and examine the fundamental facts on which I base my con- 

 clusions. But it is equally necessary to connect the detached facts, and 

 estimate their bearing on the whole. He who in the world of organisms sees 

 only disconnected existences, in which some organic similarities appear as 



